Many modern technological applications require precise frequency standards or clocks. For example, very precise navigational standards depend on clocks of extremely high accuracy. Atomic frequency standards form the basis for many such systems. One class of atomic beam standard that has found wide acceptance is based on a cesium beam tube. Cesium beam units are the present basis for most of the national standards of frequency and time. These standards are accurate to a about 1 part in 10.sup.13.
Cesium beam standards utilize quantum effects arising from the nuclear magnetic hyperfine structure in the ground state of the cesium atom. The transition normally utilized arises from the electron-spin, nuclear-spin interaction. The transition in question is relatively insensitive to external influences such as electric and magnetic fields. This transition defines a frequency in the microwave region of the spectrum at 9,192,631,770 Hz.
Basically, the cesium beam tube provides an output that is very sensitive to the frequency of a microwave source which applies energy to the tube. The microwave source is tuned until the output of the tube is maximized. When this condition is satisfied, the frequency of the microwave source will be related in a known manner to the above-described transition frequency.
An ideal cesium beam tube operates as follows. A collimated beam of cesium atoms is passed through a magnetic state selector which selects cesium atoms in a first energy state. The selected atoms then traverse a microwave cavity in which the atoms absorb energy from or give energy to the microwave source. The absorbed or delivered energy causes some fraction of the atoms to make a transition to a second energy state. The number of atoms that made the transition is then determined in an analyzer. The frequency of the microwave source is continuously adjusted in a servo loop to maximize the output of the analyzer.
Prior art cesium beam standards differ from this ideal system. First, for the idealized system to function properly, the amplitude of the microwave radiation in the cavity must remain constant as the frequency of the microwave source is shifted in the search for the maximum of the tube output. Microwave cavities have resonances which depend on the physical structure of the cavity. These resonances are equivalent to a frequency dependent filter which alters the amplitude of the microwave signal in the cavity as the frequency is shifted. In addition, small variations in the intensity of the microwave source are difficult to completely eliminate. For example, such variations can result from temperature sensitive components in the RF chain that drives the microwave cavity. These variations can also alter the amplitude of the signal from the cesium beam tube. The changes in the cesium beam tube output which result from the variations in the intensity of the microwave signal may be mistaken by the servo loop for a shift in oscillator frequency. In this case, the servo loop attempts to correct for the frequency shift by shifting the microwave frequency, thereby introducing an error into the microwave frequency.
A second problem encountered in prior art cesium beam tubes is referred to as "Rabi pulling". The idealized system assumes that all of the atoms entering the microwave cavity are in the same energy state. In practice, there are atoms in several atomic states. The atoms in states having spectral lines adjacent to that of the desired transition give rise to a background signal in the region of the desired spectral line. This background signal is not constant with frequency; hence, it distorts the shape of the desired spectral line. This distortion shifts the position of the maximum of the tube output as a function of microwave frequency, and thus, gives rise to a second source of error in the frequency standard.
Broadly, it is the object of the present invention to provide an improved atomic beam standard.
It is a further object of the present invention to provide an atomic beam standard which is less sensitive to the variation in intensity with frequency of the microwave radiation in the microwave cavity.
It is still a further object of the present invention to provide an atomic beam standard which is less sensitive to Rabi pulling then prior art atomic beam standards.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.